84 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



Studies of actinopods utilizing sectioning techniques are 

 limited thus far to two related heliozoa that lack skeletons. 

 Actinophrys sol, examined by Wohlfarth-Bottermann (1959b; also 

 Wohlfarth-Bottermann and Kriiger, 1954) is fairly small, with a 

 single nucleus. Actinosphaerium nucleofilum (Text-fig. 4) studied by 

 Anderson and Beams (1960) is much larger and multinucleate. 

 In both, the spherical body contains a central mass of granular 

 endoplasm surrounded by hyaline, vacuolated ectoplasm. As 

 seen in electron micrographs, the ectoplasm both on the axopodia 

 and on the body is conspicuously vacuolar, consisting of a veritable 

 foam. Unlike the irregular vesicles filling the cytoplasm of 

 Amoeba and Pelomyxa, most of the vacuoles in the heliozoa appear 

 to be spherical, suggesting some turgor in the living state; some 

 authors have suggested that such vacuoles are hydrostatic devices 

 aiding in flotation. Most of them appear empty, but a class of 

 small vacuoles observed abundantly in the axopodia of Actino- 

 sphaerium contains masses of dense particles (Fig. 30, PL IX). 

 Anderson and Beams believe that these correspond to refringent 

 bodies observable with the light microscope and suggest a com- 

 parison with the nitrogenous crystals of the giant amebae. In 

 addition to the vacuoles, the ectoplasm of both genera contains 

 mitochondria and some fine tubules and minute vesicles. 



Axopodia differ from the slender, filose pseudopodia of some 

 rhizopods in possessing an axial rod or thread that is birefringent 

 in polarized light, surrounded by actively moving cytoplasm. 

 Axopodia never form a reticulum; branching may occur but is 

 inconspicuous. Contraction of the whole axopod is reported for 

 some forms. Like the reticular pseudopodia of Allogromia, the 

 axopodial surface is adhesive, and entraps particles that are 

 carried along streaming paths. Jahn and Rinaldi (1959) suggested 

 that shearing forces comparable to those postulated for Allogromia 

 may operate here between the moving peripheral cytoplasm and 

 the stationary axis. 



The axial rods of the axopodia in both Actinosphaerium and 

 Actinophrys consist of oriented bundles of packed fibrils embedded 

 in a low-density matrix surrounded by ectoplasm. In the micro- 

 graphs of Actinosphaerium (Fig. 30, PL IX) these appear as very 

 fine filaments, 6 to 12 m/x in diameter and of indeterminate length. 

 In Actinophrys they look tubular; they are roughly circular in 



